The present invention relates to a one frequency repeater for a digital radio wave transmission, and in particular, to the one frequency repeater which has high cancellation capability of a transmitter-to-receiver interference.
In a digital radio wave transmission system, a carrier, usually a microwave modulated by a digital baseband signal to be transmitted is transmitted as a digital microwave radio signal. A repeater used in the digital microwave transmission system has at least one, usually two repeating paths. Each repeating path comprises a pair of receiving and transmitting antennas. The digital microwave is received at the receiving antenna and is demodulated. The demodulated signal is applied to a modulator which modulates a carrier by the recovered digital baseband signal. Then the modulated digital microwave signal is radiated from the transmitting antenna.
The receiving antenna of one repeating path and the transmitting antenna of the other path are mounted side by side on a front face of the repeater, while the transmitting antenna of the one repeating path and the receiving antenna of the other being arranged side by side on the opposite back face of the repeater.
In the repeater, transmitting signals from the two transmitting antennas may interfere the desired signal received at each one receiving antenna, due to the side-to-side and front-to-back antenna couplings.
In order to avoid such a transmitter-to-receiver interference, a conventional repeating system uses a pair of different transmitting and receiving carrier frequencies in all hops over and over again. This system is called a two frequency repeating system.
In the view point of the frequency utilization efficiency, the two frequency repeating system is inferior to a one-frequency repeating system where the same frequency carrier pair is used for both transmitting and receiving carriers in common for all hops. In order to realize the one frequency repeating system, the transmitter-to-receiver interference must be eliminated. In order to eliminate such an interference, various proposals have been made. However, the repeaters in the conventional proposals are unsatisfactory in elimination of the transmitter-to-receiver interference for the following reasons.
(1) In the general arrangement of the conventional repeater, a signal received by a receiving antenna is supplied to a receiver. An output signal of the receiver is added in an adder to an interference cancellation signal output from a variable frequency characteristic filter to eliminate a transmission signal component leaking from the transmitting antenna to the receiving antenna. The signal free from the leakage component of the transmission signal is supplied to a demodulator. An output from the demodulator is supplied to a modulator, and an output from the modulator is supplied to the transmitter and a transversal filter which generates the interference cancellation signal in the variable frequency characteristic filter. The variable frequency characteristic filter comprises a correlation detector for detecting a correlation coefficient representing a correlation between the transmission signal and the reception signal and supplies a weighting control signal determined by the correlation coefficient to the transversal filter, thereby controlling the interference cancellation signal. Since the transmission and reception signals include an identical baseband signal, the correlation between the reception signal and the transmission signal component leaking into the reception signal via the receiving antenna is excessively strong to disable accurate correlation detection. Therefore, the magnitude of the interference cancellation signal to be supplied to the adder, and hence proper interference cancellation cannot be performed in the adder.
(2) The variable frequency characteristic filter is designed to correspond to the interference signal along a variable interference route. However, the leakage of the transmission signal into the reception signal is present from the modulator output point as an interference signal determined by an inherent frequencies of the transmitter and the receiver. The function of the variable frequency characteristic filter must include a correction function for correcting the inherent frequency characteristics in addition to the above-mentioned function for compensating the variable interference route. As a result, the correction function inherent to the variable frequency characteristic filter is impaired.
(3) In conventional one frequency repeater, a means for setting a transmission signal carrier frequency is associated with both a frequency of a carrier signal generated by a carrier generator and a frequency of a local oscillated signal generated by a local oscillator. The carrier frequency of the transmission signal does not strictly coincide with the carrier frequency of the reception signal due to actual deviations or temporal variations in these frequencies. For this reason, the leakage of the transmission signal into the reception singal generates a beat frequency component. This beat frequency component is generated regardless the phase of the carrier signal of the reception signal. Even if the amplitude and phase distortion correction functions by an equalizer in a demodulator are used, the beat frequency component cannot be eliminated. Therefore, high circuit quality of the radio communication system cannot be maintained due to an interference caused by leakage of the transmission signal into the reception signal.